In a power electronics apparatus, it is necessary to switch back and forth between execution and stop of power supply in order to drive a load such as an electric motor. To this end, a semiconductor switching element such as a metal-oxide-semiconductor field-effect transistor (MOSFET) or an insulated gate bipolar transistor (IGBT) is used. A current path of a semiconductor switching element is composed of a vertical structure or a lateral structure. In a vertical structure, a withstand voltage is ensured in a vertical direction, that is, a thickness direction, so that a high withstand voltage can be easily obtained. As a structure for a switching element, a structure having an insulated gate is employed in many cases, and there are two types of a planar switching element and a trench-gate switching element. A trench-gate switching element has an improved channel density, and so, can easily obtain low on-resistance. Such being the case, a vertical and trench-gate switching element is employed in many cases. As a semiconductor material for a switching element, silicon (Si) is typically used, and in recent years, also a wide-band-gap semiconductor typified by silicon carbide (SiC) is used.
International Application Laid-Open No. WO98/35390 (A1) (Patent Document 1) discloses a vertical and a trench-gate MOSFET using SiC. On a wall portion and a bottom portion of a trench of the MOSFET, a trench oxide (gate insulating film) is provided. In a region below the gate oxide film, a protective region (field relaxing region) of a conductivity type reverse to a conductivity type of each of a source and a drain, is provided. The foregoing document suggests that the protective region protects the gate oxide film from degradation or dielectric breakdown which is likely to be caused by a high voltage applied to the drain.
As described above, known is a technique of providing a field relaxing region for protecting a gate insulating film in a vertical and trench-gate switching element. Because of such a configuration, a depletion layer extends from a field relaxing region toward a drift layer in an off state of a switching element. This depletion layer can reduce an electric field applied to a gate insulating film on a bottom face of a gate trench. This behavior is particularly useful for a switching element using SiC, as compared to a switching element using Si. This is because, in a case where SiC is used, dielectric breakdown of a switching element is more likely to occur in a gate insulating film than in an SiC region being as a semiconductor region. The reason for this is that an avalanche field intensity of SiC is approximately ten times as high as that of Si. Thus, in a case where SiC is used as a semiconductor region, a gate insulating is likely to be subjected to dielectric breakdown earlier than a semiconductor region.
For a gate trench located inside an active region of a semiconductor element, the above-described field relaxing effect can be attained from not only a field relaxing region provided therein, but also a field relaxing region provided in another gate trench adjacent thereto. Meanwhile, for a gate trench located in an outermost edge of an active region, such additional effect as described above cannot be attained. Accordingly, there is vulnerability in that a gate insulating film is easily broken down in an outermost edge of an active region.
International Application Laid-Open No. WO2015/015808 (A1) (Patent Document 2) discusses a structure for a semiconductor device which has been devised considering the above-described problem. More specifically, a trench is provided in an active region, and further, a terminating trench is formed so as to surround the foregoing trench in a terminating region in a perimeter of the active region. A protective diffusion layer (field relaxing region) is provided below not only the trench in the active region, but also the terminating trench. As a result of this, the above-described vulnerability in a gate trench located in an outermost edge of an active region can be overcome. The protective diffusion layer may be connected to a source electrode via a contact hole, and effects in this case are described in Patent Document 2 as follows.
Upon a switching operation of a silicon-carbide semiconductor device, an on state and an off state alternate in a switching period. In an off state, carriers are diffused from a protective diffusion layer so that a depletion layer extends, while in an on state, the diffused carriers return back to an original state. If carriers are slow in returning at a time of switching to an on state, a switching speed is correspondingly reduced, to cause an increase of switching loss. Because of connection of the protective diffusion layer to the source electrode, the carriers are drawn back to the protective diffusion layer by virtue of a source potential, so that switching loss is reduced.